U.S. Pat. No. 3,842,194 of Jon K. Clemens disclosed a video disc having a playback system utilizing variable capacitance. In one configuration of the Clemens system information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths about 3.5 micrometers and groove depths of about 1.0 micrometer may be used. During playback a pickup stylus about 2.0 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick, engages the groove as the record is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the pre-recorded information.
In the system of the above type the use of a relatively fine record groove and the groove engaging requirements for the pickup stylus results in a stylus tip which is extremely small. In U.S. Pat. No. 4,162,510 of Keizer, entitled "Keel-Tipped Stylus for Video Disc Systems", a novel keel-tipped pickup stylus structure is disclosed.
The pickup stylus is part of the signal pickup assembly of a video disc record player. The assembly is housed in a protective housing such as the pickup cartridge described in Leedom, U.S. Pat. No. 3,952,147, which also facilitates replacement of the signal pickup assembly without any critical manual adjustments or connections.
Dholakia, in U.S. Pat. No. 4,077,050 incorporated herein by reference, describes a leaf spring (flylead) for urging the stylus into the record groove. One end of the leaf spring is secured to the stylus and the second end is mounted within the cartridge above the stylus. The leaf spring provides stylus tracking pressure, properly locates the free end of the stylus arm relative to the carriage and establishes electrical connection between the electrode on the stylus and the signal processing circuitry of the player.
The flyleads are thin, about 0.0002 to about 0.0008 inch (5.08-20.32 micrometers) thick, about 0.04 to 0.01 inch (0.09-0.03 cm) wide and about 1 inch (2.5 cm) long. The flyleads are generally fabricated out of an electrically conductive metal.
Small items can be fabricated by ordinary photolithographic processes which employ a photoresist and include development and etching steps. A pattern is recorded in the photoresist by exposure to light through an appropriate mask. The unwanted photoresist is removed by selectively dissolving the photoresist in a solvent. The resulting pattern is then transferred to the underlying substrate material by an etching technique. The photoresist must be resistant to attack by the etchant.
The two principal types of photoresists are positive working and negative working. If a positive working photoresist is used, the exposed areas become more soluble in a solvent, called the developer solvent, than the unexposed areas. Thus, in the development step the developer removes photoresist from the exposed region leaving the bare substrate. If a negative photoresist is used, the exposed regions become less soluble in the developer. Therefore, in the development step the developer removes the photoresist from the unexposed areas, baring the substrate.
The uncovered substrate is then etched with an etchant which preferentially attacks the substrate rather than the photoresist, thereby transferring the pattern to the substrate. Finally, the remaining photoresist is removed by an appropriate solvent.
Thin beryllium copper alloy sheets can be used to fabricate the flyleads. The sheets are fragile and cannot be easily handled because they easily bend and kink. Furthermore, standard photolithographic methods were found not to be effective. For example, severe undercutting occurs at moderate bath temperatures (130.degree. F. (54.degree. C.) ). Undercutting is an attack by an etching or developing solution upon the substrate walls underneath the photoresist layer, or the photoresist walls beneath the insoluble surface photoresist layer, respectively. Undercutting results in a surface pattern having sloped walls rather than the desired straight walls.
It would, therefore, be desirable to have a process for preparing flyleads which overcomes these handling and processing difficulties.